Argon is a highly inert element over a very wide range of conditions, both at cryogenic and very high temperatures. It is used in steel-making, light bulbs, electronics, welding and gas chromatography. The major source of argon is that found in the air and it is typically produced therefrom using cryogenic air separation units. The world demand for argon is increasing and thus it is essential to develop an efficient process which can produce argon at high recoveries using cryogenic air separation units.
The most significant increase in argon production can be realized for cases where the air separation unit is operated at an elevated pressure (i.e., a feed air pressure greater than 100 psia). Using the conventional air separation schemes at the higher pressures, the argon recovery becomes very low since the argon/oxygen separation becomes more difficult at higher pressures. The focus of the present invention is for the recovery of argon at elevated pressures.
Historically, the typical cryogenic air separation unit used a double distillation column with a crude argon (or argon side arm) column to recover argon from air. A good example of this typical unit is disclosed in an article by Latimer, R. E., entitled "Distillation of Air", in Chemical Engineering Progress, 63 (2), 35-59 [1967]. A conventional unit of this type is shown in FIG. 1, which is discussed later in this disclosure.
However, this conventional process has some shortcomings. U.S. Pat. No. 4,670,031 discusses in detail these shortcomings and explains the problems which limit the amount of crude argon recovery with the above configuration. This can be easily explained with reference to FIG. 1. For a given production of oxygen and nitrogen products, the total boilup and hence the vapor flow in the bottom of section I of the low pressure column is nearly fixed. As this vapor travels up the low pressure column it is split between the feed to the crude argon column and the feed to the bottom of section II of the low pressure column. The gaseous feed to the top of section II of the low pressure column is derived by the near total vaporization of a portion of the crude liquid oxygen stream in the boiler/condenser located at the top of the crude argon column. The composition of this gaseous feed stream is typically 35-40% oxygen. A minimum amount of vapor is needed in section II of the low pressure column, namely the amount necessary for it to reach the introduction point of the gaseous feed to the top of section II without pinching in this section. Since the composition of the gaseous feed stream to the top of section II is essentially fixed, the maximum flow of vapor which can be sent to the crude argon column is also limited. This limits the argon which can be recovered from this process.
In order to increase argon recovery, it is desirable to increase the flow of vapor to the crude argon column. This implies that the vapor flow through section II of the low pressure column must be decreased (as total vapor flow from the bottom of the low pressure column is nearly fixed). One way to accomplish this would be to increase the oxygen content of the gaseous feed stream to the top of section II of the low pressure column because that would decrease the vapor flow requirement through this section of the low pressure column. However, since this gaseous feed stream is derived from the crude liquid oxygen, its composition is fixed within a narrow range as described above. Therefore, the suggested solution is not possible with the current designs and the argon recovery is thus limited.
Recently, elevated pressure (EP) cycles have been proposed for air separation plants. In the EP cycles, the supply pressure of air to the cold box is higher than the conventional pressures of 80-95 psia. Typically, these pressures are higher than 100 psia. One key advantage is that at a higher pressure, smaller equipment is required due to the smaller volume of flow. In addition, significant power savings can be realized when high pressure products are desired. By operating the air separation unit at an elevated pressure, the pressure of streams sent to the product compressors also increases. This reduces the pressure ratio across the product compressors which translates to significant power savings. This power reduction more than offsets the additional power required to compress the column air to the elevated pressure. A key disadvantage of operating the air separation unit at an elevated pressure, however, is that the argon recovery is usually very low. This is due to the difficulty of the Ar/O.sub.2 separation at the higher pressures.
To increase the argon recovery for the EP cycle, U.S. Pat. No. 5,034,043 suggests operating the crude argon column at a lower pressure than the one dictated by the feed from the low pressure column. The rationale is that by operating at the lower pressure, the separation of argon and oxygen becomes less difficult and hence, more argon can be recovered. The scheme involves expanding the crude argon column feed from the low pressure column prior to the crude argon column. The separation is then done at a reduced pressure. The bottom stream from the crude argon column is then boosted in pressure by a pump and returned to the low pressure column. The disadvantage of this method is that the amount of feed to the crude argon column is still limited. Furthermore, the difficulty of the Ar/O.sub.2 separation still exists in the low pressure column which also restricts the concentration of argon in the feed sent to the crude argon column. Overall, the amount of argon recovery is still very limited. Another deficiency of this scheme is that crude liquid oxygen from the bottom of the high pressure column which is vaporized at the top of the crude argon column is at a pressure lower than the low pressure column. Therefore, this vaporized stream is warmed, boosted and recycled to the low pressure column. This adds another booster compressor and adds recycle losses. The recycle flow is a substantially large fraction of the feed air.
U.S. Pat. No. 4,822,395 teaches another method of argon recovery. In this method all the crude liquid oxygen from the bottom of the high pressure column is fed to the low pressure column. Instead of drawing all the oxygen product as gaseous oxygen from the low pressure column, nearly all the oxygen product is withdrawn as liquid oxygen from the bottom of the low pressure column, reduced in pressure and boiled in the boiler/condenser located at the top of the crude argon column. The crude argon column overhead vapor is condensed in this boiler/condenser and provides reflux to this column. It should be noted in this patent that all the condensing duty for the reflux at the top of the crude argon column is provided by vaporizing liquid oxygen from the bottom of the low pressure column. There are some disadvantages to this method also. The liquid from the bottom of the low pressure column is nearly pure oxygen. Since it condenses the crude argon overhead vapor, its pressure when boiled will be much lower than the low pressure column pressure. This means that nearly all of the oxygen gas recovered will be at a pressure which is significantly lower than that of the low pressure column. When oxygen is a desired product, this leads to a higher energy consumption due to the lower suction pressure at the oxygen product compressor. Another drawback of the suggested solution is that since crude argon overhead is condensed against pure oxygen, the amount of vapor which can be fed to the crude argon column is limited by the amount of oxygen present in the air. Consequently, even though the vapor flow is increased in the bottom section of the low pressure column by not drawing any gaseous oxygen, the feed to the crude argon column still has to be quite low. The recovery of argon is therefore severely limited.
Finally, another process teaching a method to improve argon recovery is taught in U.S. Pat. No. 5,114,449. This prior art process is shown in FIG. 2 which is also discussed later in this disclosure. In this process, all the crude liquid O.sub.2 from the bottom of the high pressure column is fed to the low pressure column. The vapor at the top of the crude argon column is now condensed by heat exchange with a liquid stream in the low pressure column. This heat exchange place is located between the crude liquid oxygen feed location and the withdrawal point of the argon-rich vapor stream which is the feed stream for the crude argon column. This thermal linkage between the crude argon and the low pressure columns leads to enhanced argon recovery when compared to the process shown in FIG. 1 and the one taught in U.S. Pat. No. 4,670,031. However, in certain instances, this enhanced argon recovery is still not sufficient to meet the increased demand of argon and it is desirable to envision methods which would further increase the argon recovery.
Clearly then, there is a need for a process which does not have the above-mentioned limitations and can produce argon with greater recoveries.